1. Field of the Invention
The present invention relates to a surface emitting laser device, a surface emitting laser array, an optical scanning device, and an image forming apparatus. More particularly, the present invention relates to a surface emitting laser device and a surface emitting laser array capable of emitting light in the direction orthogonal to their substrate, an optical scanning device including the surface emitting laser device or the surface emitting laser array, and an image forming apparatus having the optical scanning device.
2. Description of the Related Art
A Vertical Cavity Surface Emitting Laser (hereinafter may be referred to as “VCSEL”) is capable of emitting light in the direction orthogonal to its substrate. When compared with edge emitting semiconductor lasers capable of emitting light in the direction parallel to its substrate, the VCSEL may have some advantages such as lower cost, lower energy consumption, smaller size, preferable to two-dimensionally integrated devices, and higher performance. Recently, because of those advantages, the VCSEL has attracted increased attention.
The application fields of the surface emitting laser device include a light source (oscillation wavelength: 780 nm band) for an optical writing system in a printer, a writing light source (oscillation wavelength: 780 nm band and 850 nm band) in an optical disk apparatus, and a light source (oscillation wavelength: 780 nm band, 850 nm band, 1.3 μm band, and 1.5 μm band) in a light transmission system such as an LAN (Local Area Network) using optical fibers. Further, a surface emitting laser device is expected to be used as a light source for optical transmissions between boards, within a board, between chips in a Large Scale Integrated Circuit (LSI), and within a chip in the LSI.
In those application fields, it is generally desired that the light emitted from the surface emitting laser device (hereinafter referred to as “emitting light”) has a high-power single lateral mode. Especially, there is a strong demand for high power in the fundamental lateral mode oscillation. To that end, it is required to suppress the high-order lateral mode oscillation, and various efforts have been made.
For example, Japanese Patent Application Publication No. 2001-156395 (Patent Document 1) and U.S. Pat. No. 5,940,422 (Patent Document 2) propose a configuration having an upper electrode and a mode-selection filter, the upper electrode being provided on an emission surface through which the laser light is emitted and being provided so as to surround the emitting region, the mode-selection filter being made of a transparent dielectric film designed so that the reflectance of the center part is different from that of the surrounding part. Specifically, in some cases, the reflectance of the center part is set greater than that of the surrounding part in the emitting region, so as to facilitate the fundamental lateral mode operation to increase the power of fundamental mode output. In other cases, the reflectance of the surrounding part is set greater than that of the center part in the emitting region, so as to facilitate the high-order lateral mode operation.
However, in the configurations disclosed in Patent Documents 1 and 2, when the adhesion between the filter and the contact layer is not sufficient, the filter may be peeled off. Further, due to the gap between the filter and the upper electrode, the emission surface and the contact layer may be contaminated or oxidized.